This application is a 371 of International Application No. PCT/EP99/06498, filed Sep. 3, 1999, which in turn claims the priority of German Applications Nos. 198 40 527.8, filed Sep. 6, 1998, and 198 49 048.8, filed Oct. 23, 1998.
(1) Field of the Invention
The invention relates to suspensions and powders based on indium tin oxide, methods of preparing them, mouldings produced from them, and also coating and moulding compositions and the use thereof as coating materials.
(2) Description of Related Art
Optoelectronic properties are a characteristic of indium tin oxides. In the form of thin transparent layers, for example, the oxides are able to reflect infrared light and at the same time combine a relatively high electronic conductivity with transparency in layer systems. For this reason, there are a very large number of possible uses for indium tin oxide (ITO) systems, and, accordingly, there have also been a very large number of investigations into their preparation.
The most common methods of applying transparent layers are gas phase techniques, in which the ITO is deposited from the gas phase onto the substrate in the form of a thin coherent layer. Other methods used include the sol-gel process, or powder and paste technologies.
A characteristic of the indium-oxygen system is the large number of compounds. The most thermodynamically stable is In2O3. Indium oxides of composition In4O3, In4O5, In2O and In7O9 are usually formed by reduction of In2O3 in a stream of hydrogen. At room temperature, In2O3 is dark yellow to pale yellow, at higher temperatures it is brown to brownish red, and it is soluble in mineral acids. Only the cubic modification can be detected by rontgenography.
For the preparation of pure indium oxide powders, the literature describes predominantly precipitation from solutions. The chosen hydroxides are subsequently converted into the oxides by calcining. Aqueous salt solutions are precipitated with alkali metal solutions, with ammonia or with urea; see, for example, JP 06227815 A2, JP 05193939 A2, JP 04325415 A2, JP 04219315 A2 and DE 2127135 A.
Occasionally, precipitation is also carried out in the presence of sulphuric acid or sulphate solutions; see, for example, JP 05058627 A2. However, no information is given on the effect of the precipitate on the quality of the powder. Additionally, the information given by the literature regarding particle size or agglomeration state, if given at all, is very imprecise. The particle sizes, which are generally calculated back from the measurement of the BET surface area of the powders, extend from the nanometer range up into the region of 100 xcexcm.
Indium oxide is a wide-gap n-semiconductor whose intrinsic electronic conduction derives from oxygen defects. Besides intracrystalline effects, the mobility of the charge carriers is restricted in particular by the hindrance of intercrystalline electron transitions. One possibility for increasing the low charge carrier density of plain indium oxide is the specific incorporation of tetravalent elements such as, for example, tin.
Various ways of preparing indium oxide/tin oxide mixtures are known. In the case of the simple mixed oxide method of preparing ITO mixtures, temperatures between 700xc2x0 C. and 900xc2x0 C. are required; see, for example, EP 654447 A1. The sol-gel technique is suitable likewise for preparing ITO mixtures, with specific powder surface areas of 10 m2/g being indicated; see, for example, JP 06293517 A, JP 06080422 A and JP 05201731 A. There are also descriptions of electrolysis methods, in which anodic oxidation of an indium electrode or of an indium tin electrode produces hydroxides, which are converted into oxides by subsequent calcining; see JP 63195101 A2, JP 06171937 A2 and JP 06329415 A2. Furthermore, indium tin hydroxides are dispersed in organic solvents, dehydrogenated by azeotropic distillation, and then converted into the oxides by drying and calcining; see JP 02006332 A2. ITO powders may also be prepared by an arc discharge between a tungsten electrode and an indium electrode in an argon/oxygen mixture (Y. Endo et al., Funtai, Kogaku Kaishi (1995), 32 (12), 874-80) or by means of aerosol spray pyrolysis of indium acetate in water in an argon carrier gas (D. M. Speckmann et al., Mater. Res. Soc. Symp. Roc. (1995), 372 (Hollow and Solid Spheres and Microspheres; Science and Technology Associated with Their Fabrication and Application), 247-52, or by spraying indium tin salt solutions at 800xc2x0 C. (JP 01087519 A). Indium oxide or tin oxide may likewise be prepared by condensing indium chloride and tin chloride from the gas phase followed by reactions with oxygen or water (JP 05024836 A2), and by corona discharge in a reductive atmosphere at 1000xc2x0 C. (DE 4407774 C1).
To prepare ITO layers, ITO powders are used directly, for example. For example, JP 07118840 A uses an ITO powder having a specific surface area of 30 m2/g, JP 06049394 A an ITO powder having a diameter of 200 nm, and JP 05036314 A an ITO powder having an average particle size of 30 nm.
All in all, this means that there are a large number of preparation methods for tin-doped indium oxide (ITO) powders. Defined information regarding powder qualities in conjunction with their mode of preparation, however, is not described. The quality of the powders used is normally defined by way of the application. In recent years, there has been a sharp increase in interest in a defined preparation and application of transparent conductive layers on various substrates. Whereas the use of Sb- and F-doped tin oxide layers on glass substrates has been known for some time on the basis of their conductivity, their transparency in the visible spectral range and their reflection properties in the IR range, for the surface heating of glasses for aircraft, spaceships and cameras and also for electrostatic shielding purposes, diverse requirements have recently come about regarding such layers for uses in microelectronics and optoelectronics. Examples of such uses include
1. transparent driver electrodes for liquid crystal displays, thin-film electroluminescent displays and electrochromic displays
2. transparent conductive layers for highly sensitive radiation detectors, ferroelectric photoconductors and memory systems
3. transparent conductive oxide films as gate electrodes for charge, injection and charge-coupled systems.
These uses in optoelectronics are at the same time tied to more stringent requirements regarding conductivity, transparency and structurability of the layers. Because of the unfavouring structuring properties of doped tin oxide layers in connection with customary structuring by chemical etching technologies, it is predominantly tin-doped indium oxide layers (ITO) which have become established for these uses.
Furthermore, these ITO layers have a significantly better conductivity and transparency than doped tin oxide layers. Tin-doped indium oxide layers are currently the most conductive coatings available commercially. In routine operations, the specific resistance which can be achieved is approximately 1-2xc3x9710xe2x88x924 ohm.cm, which in conjunction with an approximately 30 nm thick barrier layer of SiO2 leads to a surface resistance of 15 xcexa9/xe2x96xa1 at a layer thickness of just 120 nm (transparency  greater than 90%). Because of preparation by sputtering or CVD techniques, the costs for this type of coating are comparatively high, and extensive coatings are difficult to implement.
The high charge carrier density in conjunction with a charge carrier mobility in the range of 40-60 cm2/Vs leads to very high transparency in the visible region with outstanding reflection in the IR region at the same time. The fraction of tin oxide is usually between 7-12% by weight.
For many applications, especially in microelectronics and optoelectronics, with optical and IR-blocking coatings it is important to use ITO powders comprising nanoscale particles. Such nanoscale particles have an average particle size of preferably not more than 200 nm, in particular not more than 50 nm, and with particular preference not more than 30 nm. A particularly preferred range is from 5 to 30 nm.
In many methods of preparing indium tin oxide powders or suspensions, suspensions of precursors of the indium tin oxide powder are obtained from which the indium tin oxide powders, after drying, are prepared by calcining. The particle sizes of the powders thus prepared, however, are not within the desired nm range, and the powders also cannot be reprocessed to suspensions having particle sizes in the nm range (nanoscale particles), so that with low-viscosity suspensions prepared in this way it is not possible to use dipping, spraying or similar processes to produce coatings having particle sizes in the nm range.
Although nanoscale primary particles of suitable size are formed during the formation of the ITO powder precursors in suspension under the appropriate conditions, these primary particles undergo aggregation to larger particles in the course of processing, especially in the course of calcining. The reason for this aggregation is that a decrease in particle size is accompanied by the domination, or considerable increase in significance, of weak forces of interaction, such as van der Waals forces, for example. A further factor is that the particle surface is always covered with functional, i.e. condensable, groups which lead to the formation of hard aggregates as a result of condensation reactions between individual primary particles. In that case, the primary particles are connected to one another by way, so to speak, of sintering necks. These aggregates of primary particles can no longer be disrupted by a simple dispersing step in an aqueous or nonaqueous solvent.
Powders of this kind, prepared by calcining, cannot be used for highly transparent layers, since even particles with sizes of 50 nm lead to optical defects.
It was therefore an object of the present invention to prepare compositions based on indium tin oxide in such a way that, despite calcining, nanoscale particles of suitable size are obtained and the powders can be processed in a simple dispersing step to give suspensions, with full retention of the primary particle size.
This is made possible in accordance with the invention by a method of preparing suspensions and powders based on indium tin oxide, characterized in that
a) indium tin oxide precursors are precipitated from solutions of indium compounds and tin compounds in one or more solvents in the presence of one or more surface-modifying components,
b) the solvent is removed from the resulting powder, which is then calcined,
c) one or more surface-modifying components and one or more solvents are added,
d) the resulting mixture is subjected to a comminuting or dispersing treatment, with formation of a suspension, and
e) any liquid components are separated from the suspension, to give a powder.
Furthermore, it is possible, if desired, to convert the suspension or the powder produced from it into a moulding by way of a shaping process.
The method of the invention results, surprisingly, in suspensions and powders based on indium tin oxide which consist substantially of primary particles and which contain substantially no agglomerates formed by aggregation of primary particles. The powders can easily be redispersed to give suspensions in which the primary particle size is almost fully retained.